Most integrated circuits (IC) make use of current references. These current references can be realized, for example, by biasing a transistor in a saturation region. The resulting circuit is very simple, requires a small area and can operate with minimum battery voltage. The main limitation to this solution is accuracy of the reference current. The current generated by the transistor cannot be controlled more accurately than the fabrication/temperature spread of the saturation current.
For better accuracy a Band-Gap (BG) based biasing circuit may be used, but this solution also has disadvantages. If it is supplied by a Charge pump, the BG-based solution has a high cost due to the BG current consumption and the resulting impact on the size of the charge pump required to support the BG. It is undesirable to use chip real estate for a large charge pump. An alternative solution is to provide a battery to supply the BG from battery. If a battery is used, then the current consumption of the block is less significant on chip design. Even if a battery is used, a critical issue remains—how to transfer the reference current to the charge pump voltage domain.